Quadrupole ion traps according to Paul and Steinwedel (German patent DE-PS 944 900) consist of ring and end cap electrodes between which an essentially quadrupolar storage field is generated by applying RF voltages to the ring and end caps. Ions with varying mass-to-charge ratios (m/q) can be stored at the same time in this field (for the sake of simplicity, only "masses" instead of "mass-to-charge ratios" are referred to in the following since, in ion traps, one is predominantly only concerned with singly charged ions).
Physically intrinsic resonance conditions of the storage field are preferably used for ion ejection. With a pure quadrupole field, resonance conditions of this kind are found at the edge of the stability zone in the a,q diagram. In addition, with certain nonlinear conditions, in particular, those which occur in the case of a superposition of multipole fields, resonance conditions occur inside the stability zone and can also be used for ion ejection.
FIG. 1 shows some known storage field resonance conditions for a pure quadrupole field and for superposed hexapole and octopole fields plotted on an a,q stability diagram. The storage field resonances, .beta..sub.z =1 (for pure quadrupole), .beta..sub.z =2/3 (for hexapole superposition), .beta..sub.z +.beta..sub.r =1 and .beta..sub.z =1/2 (both for octopole superposition), have been plotted. The following applies in the customary manner: EQU a=-8zU/(mr.sub.0.sup.2 .OMEGA..sup.2), q=4zV/(mr.sub.0.sup.2 .OMEGA..sup.2)
where:
z=Coordinate of the rotationally symmetric axis of the ion trap, PA1 U=Direct voltage with which the RF storage field is superposed, PA1 m=Mass of ions, PA1 r.sub.0 =Inside radius of the ring electrode, PA1 .OMEGA.=Angular frequency of the storage RF, and PA1 V=Amplitude (voltage) of the storage RF
The advantages of these superposed multipole fields are discussed in detail in the International Journal of Mass Spectroscopy Ion Processes, J. Franzen, v. 106, pp. 63-78 (1991) which article is hereby incorporated by reference.
For measurement of the spectra, the ions are brought to a resonance condition of this kind mass by mass by changing the amplitude of the quadrupole RF storage field. When ions of a particular mass reach the resonance condition, they absorb energy from the RF storage field, enlarge their oscillation amplitudes and leave the ion trap through small holes in one of the end caps. The ejected ions can then be measured outside the ion trap with an ion detector.
The secular oscillation frequency of the ions varies widely after their production or introduction into the trap. Consequently, in order to provide a well-resolved mass spectrum, it is necessary to first collect the oscillating ions confined in the ion trap near the center of the ion trap to enable the ions of successive masses to leave the ion trap in ejection cycles clearly separated from each other in terms of time. For this, the ion trap is preferably filled with a special damping gas having an optimal density enabling the ions to release energy by colliding with the remaining gas in the trap. When such a gas is introduced, the trapped ions "thermalize" after a few collisions and collect at the center of the quadrupole field due to the focusing effect of the quadrupole field, reducing their oscillation amplitudes at the same time. They form a small cloud, the diameter of which is only approximately 1/20 to 1/10 of the dimensions of the trap according to tests carried out with laser beams as described in Physical Review A, I. Siemers, R. Blatt, T. Sauter and W. Neuhauser, v. 38, p. 5121 (1988) and Journal of the Optical Society of America B, M. Schubert, I. Siemers and R. Blatt, v. 6, p. 2159 (1989). Thermalization takes place particularly quickly with medium-weight damping gas molecules such as air.
The absorption of energy under the resonance condition physically built into the storage field necessarily assumes, however, that the ions are not in a state of calm at the center of the quadrupole field since the field intensity as well as the condition of resonance disappear there. Absorption of energy due to the physically intrinsic resonance is only possible further away from the field center and increases as the ions move further from the center due to oscillations.
It is therefore beneficial to intentionally weakly excite the secular oscillation of the ions shortly before they are brought to the resonance condition. This excitation is produced by bringing the ions into resonance with a relatively weak RF excitation voltage connected via the two end caps to produce an effective field at the center of the ion trap. Only this initial coherent excitation of the ions of a particular mass enables them to absorb energy from the RF storage field in the further course of the scanning process when they reach the resonance condition. This energy absorption causes the ions to be exponentially accelerated and thus ejected from the ion trap.
Methods are already known of removing ions from the ion trap in resonance solely by the effect of the applied excitation RF voltage, for example as described in G. Rettinghaus, Z. f. Angew. Physik 22, 321, 1967. However, when the excitation voltage alone is used for ion ejection, the absorption of energy essentially leads to a linear rise in secular ion oscillation amplitude. This compares to an exponential increase, at least at the beginning, which results from use of built-in field resonance. Consequently, ion ejection is much sharper when the intrinsic field resonances are used and can be carried out in fewer oscillation cycles.
A simple scanning method with mass-sequential ejection of ions utilizing the limit of the stable storage range (.beta..sub.z =1) in the a,q diagram, without application of an additional excitation frequency for exciting the secular oscillation, has already been known for some time and is described in U.S. Pat. No. 4,540,884. However, a considerable improvement in the resolution of this latter method was obtained by the introduction of "axial modulation", which is a coherent excitation of the secular ion oscillation shortly before reaching the stability limit as described in EP-A1 0 350 159. The use of a nonlinear resonance .beta..sub.z +.beta..sub.r =1, produced by superposing a weak octopole field onto the quadrupole field, is similarly well-known with ejection of ions after initial pushing of the secular oscillation as described in European patent applications EP-A1 0 336 9901 and EP-A1 0 383 961.
With respect to ion ejection, the nonlinear multipole resonance conditions and the resonance on the stability margin differ only in so far as the multipole resonances each show sharply defined singularities (mathematical poles), while the stability margin, .beta..sub.z =1, of the quadrupole field sharply separates two large areas, one stable and the other unstable. In both cases, however, the ions experience conditions under which they are able to absorb oscillation energy from the storage field.
If even multipoles are involved (octopoles, dodecapoles etc.), the singularities in the stability zone by no means represent points of instability, but only points for limited absorption of energy, since the secular frequency of the resonating ions changes with increasing amplitude and thus no enduring resonance condition exists which is unlimited in terms of time.
Under optimal conditions, the coherent initial pushing of the secular oscillation for a particular ion type should be arranged to take place a very short time (approximately 10 to 100 microseconds) before the storage field resonance is reached so that the coherently oscillating ions of the ion cloud are not again disturbed by collisions with the remaining gas. In order to achieve this, it is necessary for the excitation voltage to have a frequency slightly lower than the storage field resonance.
The amplitude setting for this excitation RF voltage is critical. The mass-spectrometric resolution decreases both with regard to voltage amplitudes which are higher or lower than the optimum voltage amplitude. The optimum is usually set by observing the output with an oscillograph, though it is also possible to use a representation of the scan profiles by means of a computer system.
Alteration of the excitation RF amplitude causes not only a change in resolution, but also a change in the scanning function, i.e. the function m=f(A), m being the mass of the ions and A the amplitude of the storage RF, used for scanning. With increased excitation amplitude, the masses appear at the exit holes earlier since they have already received excitation energy from the end cap electrodes by the excitation RF and only have to absorb a small amount of energy from the storage field to produce ejection. Consequently, for optimal results, it must be possible to reproduce the excitation amplitude well. With fast mass scans, slight changes in the ion ejection time can amount to several units of mass on the mass scale.
Experiments have established that neither a constant amplitude of the excitation voltage nor a linear change in the amplitude during the scanning process produces an optimal resolution for all masses. Although it is possible to keep resolution at an optimum by means of a piece-wise linear control, this results in nonlinearities of the scanning function at the breakpoints between the linear parts.
There are some methods (such as ion isolation or the very fast subsequent data processing) which require as constant a mass control as possible with the amplitude of the storage RF. Methods for isolation of ions and for fragmentation need a linear and constant control of the masses with an accuracy better than 1/10 of a unit of mass.
Consequently, it is the task of the invention to create a method of scanning which combines as smooth (i.e. not only partially linear) a scanning function as possible with as good a mass resolution as possible for all masses. Here, the scanning function is defined as the dependence of the mass of the ions ejected on the voltage amplitude of the storage RF.